Recently, there has been a proliferation in broadcasting stations and general households of apparatus that digitally handle image information, and when so doing, compress images for the purpose of efficient information transfer and storage. Such apparatus compress images by implementing coding formats such as MPEG which compress information using an orthogonal transform such as the discrete cosine transform and by motion compensation.
Particularly, MPEG-2 (ISO/IEC 13818-2) is defined as a general-purpose image coding format, and is currently widely used in a broad range of applications for both professional use and consumer use. By using the MPEG-2 compression format, it is possible to realize favorable image quality by allocating a bit rate from 4 to 8 Mbps if given a standard-definition interlaced image having 720×480 pixels, for example. Additionally, it is possible to realize favorable image quality by allocating a bit rate from 18 to 22 Mbps if given a high-definition interlaced image having 1920×1088 pixels.
Although MPEG-2 has primarily targeted high image quality coding suitable for broadcasting, it is not compatible with coding formats having a bit rate lower than that of MPEG-1, or in other words a high compression rate. Due to the proliferation of mobile devices, it is thought that the need for such coding formats will increase in the future, and in response the MPEG-4 coding format has been standardized. MPEG-4 was designated an international standard for image coding in December 1998 as ISO/IEC 14496-2.
Furthermore, standardization of H.26L (ITU-T Q6/16 VCEG), which was initially for the purpose of image coding for videoconferencing, has been progressing recently. Compared to previous coding formats such as MPEG-2 and MPEG-4, H.26L is known to impose more computational demands for coding and decoding, but higher encoding efficiency is realized. Also, as a link to MPEG-4 activity, a format based on H.26L which realizes higher encoding efficiency is being currently standardized as the Joint Model of Enhanced-Compression Video Coding. As part of the standardization schedule, H.264 and MPEG-4 Part 10 (Advanced Video Coding, hereinafter abbreviated “H.264/AVC”) was internationally standardized in March 2003.
Additionally, as an extension of the above, standardization of the FRExt (Fidelity Range Extension) was completed in February 2005. FRExt includes coding tools required for business use, such as RGB, 4:2:2, and 4:4:4, as well as the 8×8 DCT and quantization matrices defined in MPEG-2. As a result, H.264/AVC can be used for image coding able to favorably express even the film noise included in movies, which has led to its use in a wide range of applications such as Blu-Ray (trademark).
With such coding and decoding processes, image data is coded in units of blocks. Also, when decoding coded data, a filter is applied and deblocking is conducted on the basis of the boundary strength and the quantization parameters, as indicated in PTL 1, for example.
However, needs are growing for coding at even higher compression rates, such as for compressing images having approximately 4000×2000 pixels, or for delivering high-definition images in an environment of limited transmission capacity such as the Internet. Thus, extending the macroblock size beyond that of MPEG-2 and H.264/AVC to a size such as 32×32 pixels, for example, has been proposed in literature such as NPL 1. Namely, with the proposal in NPL 1, by adopting a tiered macroblock structure, larger blocks are defined as a superset while maintaining compatibility with the H.264/AVC format for blocks of 16×16 pixels or less.